For home espresso you really want the ability to tweak the grind size for each shot. Small variations in grind size can significantly change the flow rate, which dictates the peak pressure & shot time you can achieve. When you buy a new bag of beans it can take a couple tries to "dial in". The appropriate grind size depends substantially on the coffee variety, roast level, age of beans, etc. If the pre-ground coffee doesn't hit the mark then you are SOL for the batch -- including if it is too fine and doesn't flow at all. Even if you hit the mark in the beginning, the shot behavior will change over time as the beans age.